The present invention relates to piezoelectric crystalline films. More particularly, it relates to piezoelectric crystalline films comprising zinc oxide having a hexagonal crystal structure.
There are many methods for making piezoelectric zinc oxide films such as, for example, vacuum deposition methods, epitaxial growth methods, and sputtering methods. Among these methods, the sputtering methods, particularly, a radio-frequency sputtering method has been used very often lately because of the advantage that the growing rate of oriented crystalline films is high, thus making it possible to mass-produce piezoelectric crystalline films, industrially.
When making a piezoelectric crystalline film of zinc oxide on a substrate surface by the radio-frequency sputtering method, ceramics of highly pure zinc oxide has been conventionally used as a source of film material. However, even when radio-frequency sputtering is effected with such a film material source, it is difficult to make piezoelectric crystalline films with a c-axis perpendicular to the substrate surface. If the c-axis of the zinc oxide film is inclined with respect to the axis perpendicular to the substrate surface, the value of the electromechanical coupling factor becomes small, thus making it difficult to produce a piezoelectric crystalline film transducer with good conversion efficiency.
It has now been found that the use of ceramics of zinc oxide containing bismuth makes it possible to produce a piezoelectric crystalline film with a c-axis perpendicular to the substrate surface and with high quality.
It is an object of the present invention to provide an improved zinc oxide piezoelectric crystalline film which overcomes the aforesaid disadvantages.
According to the present invention, there is provided a piezoelectric crystalline film of zinc oxide with a c-axis perpendicular to the substrate surface, characterized in that said crystalline film contains 0.01 to 20.0 atomic percent of bismuth.
The piezoelectric crystalline film of the present invention may be made by any conventional methods such as for example, the radio-frequency sputtering methods, the co-sputtering methods and the ion plating methods.